The difference is minimal. 98% of US beef is grass raised and finished on a grain mixture the last 6 months before processing. The nutritional profile between the two is negligible. If great taste isn’t a concern, the healthiest beef will be from other counties such as Uruguay, Australia, or New Zealand.
Beef is good for you, but grass-fed beef is definitely better compared to grain-fed or finished. It is long understood by ranchers that grain fed and/or finished beef is fattier than grass raised; has more white fat, marbling, and a different flavor. Tends to up the Omega-6s in the individual eating the beef the more grain there is as compared to grass fed beef. Long story short grain has more calories available than grass, and is thus a fattier food. Cattle evolved to thrive eating grass, so they tend to do better on grass even if you can balloon them up quickly with grain. Grain can also be a lot cheaper.